This invention relates generally to surgery, and more particularly to methods and instrumentation having their principal utility in spinal surgery.
My U.S. Pat. Nos. 5,242,443, dated Sep. 7, 1993 and 5,480,440, dated Jan. 2, 1996, the disclosures of which are here incorporated by reference, describe a surgical technique for percutaneous fixation of two or more adjacent vertebrae by means of screws inserted percutaneously into the pedicles of the vertebrae and secured together by links located just under the skin of the patient""s back. The technique avoids the many difficulties encountered in conventional internal and external vertebral fixation.
In vertebral fixation according to the methods described in my prior patents, a cannulated tubular guide is maneuvered into alignment with the pedicle. A pin is introduced through the guide and tapped with a mallet so that it enters the cortical bone at the junction of the base of the transverse process and the proximal articular process. The guide is then removed and a cannulated obturator is placed over the pin. An access cannula is then placed over the obturator and advanced to the pedicle. The obturator is then removed from the access cannula and a cannulated drill is advanced over the pin and operated to form an entrance into the medullary canal of the pedicle. A probe is then advanced into the medullary canal to create a bore into the vertebral body. The bore may then be tapped to form threads engageable by a pedicle screw, or alternatively a self-tapping pedicle screw can be inserted. With pedicle screws threaded into pedicles of adjacent vertebrae, adapters of the appropriate length are selected and secured to the proximal ends of the screws. The screws, with the adapters attached to them, are connected by links located just underneath the patient""s skin. The procedure is much less invasive than conventional internal fixation, minimizes damage to muscle tissue and ligaments, reduces recovery and rehabilitation time, and simplifies removal of the fixation appliances. The procedure also reduces the infection risks, and avoids the physical limitations, imposed on the patient by external fixation.
The placement of a plate or rod under the skin, following commonly practiced open spinal fusion and pedicular screw insertion, facilitates subsequent retrieval of the hardware when the fusion and stabilization are accomplished. This eliminates the need for a second operation for access to, and retrieval of, the deeply positioned hardware.
The internal diameter of the medullary canal of the lumbar pedicles is typically only about 7 to 8 mm. The small size of the medullary canal mandates precise positioning of screws in the pedicle. To achieve proper alignment of the cannulated tubular guide with the axis of the pedicle, the guide is visualized fluoroscopically as it is being inserted. When properly aligned, the guide appears as an opaque circle in the center of the pedicle. A similar fluoroscopic method is used for alignment of the screw with the pedicle, the screw appearing as a dot in the center of the pedicle when properly aligned. This method is referred to as the xe2x80x9cbulls-eyexe2x80x9d method.
Conventional bulls-eye alignment has proven to be less than entirely reliable for proper positioning of percutaneously inserted guides and screws for several reasons. First, the operation is carried out using a radiolucent table and a C-arm fluoroscope. To allow better visualization of the pedicles the table is tilted away from the C-arm, or alternatively, the C-arm is tilted relative to the table. The angle of tilt of the C-arm fluoroscope relative to the operating table affects the angle of the guide or screw. Therefore, accurate measurement of the necessary tilt of the C-arm to accommodate the desired angle of insertion of the guide or screw is necessary. A second deficiency of the conventional bulls-eye alignment method arises because the skin entry point plays a significant role in the proper positioning of the guide at the center of the pedicle and the insertion of the probe into the vertebral body. A third deficiency of the bullseye method is that the distance between the guide and the x-ray tube can have an effect on the position of the guide relative to the pedicle. If the x-ray tube is too close to the patient and the angle of the tilt of the C-arm has not been predetermined and measured, the peripheral x-ray beams may present a distorted view of the position of the guide. A fourth deficiency of the bulls-eye method is that, when the C-arm is tilted by 20xc2x0 to 30xc2x0, clear visualization of the boundaries of the pedicles is difficult.
Similar difficulties are encountered in positioning a biopsy cannula for taking a vertebral biopsy. An important object of this invention, therefore, is to provide a more accurate and reliable method for establishing an insertion point for a percutaneously inserted instrument for spinal surgery.
A method for determining the point for insertion of an instrument in a percutaneous spinal procedure in accordance with the invention, comprises the following steps. The patient is scanned by computed tomography, and an image is produced of an axial plane through the patient, i.e. a plane perpendicular to the long axis of the patient""s body. On that image, a desired path for insertion of a guide pin is determined, and the lateral distance from the patient""s midline to the point at which said path intersects the skin of the patient""s back is determined. By viewing the patient""s spine radiographically in the anterior-posterior direction, the skin of the patient""s back is marked directly over the midline, and marking the skin of the patient""s back is also marked with a line extending transverse to the patient""s midline in a plane corresponding to the transverse plane in which the computed tomography scan was taken. Thereafter, an insertion point is established on the patient""s back, on the transverse line, at a distance equal to the lateral distance measured on the image.
Another important object of this invention, therefore, is to provide an more accurate and reliable method for placement of a guide in the center of a pedicle in preparation for the insertion of a pedicle screw or biopsy cannula.
In accordance with the invention, the insertion points and insertion angles for the guide pins are established by a technique using a combination of computed tomography and conventional radiographic visualization. CT scans of the patient are taken in axial planes through the pedicles of two or more vertebrae to be fixated. Using the CT images, a desired path for insertion of a guide pin into each of these vertebrae is established. The angle of the path relative to the median plane is measured on the CT image for each of the vertebrae. Also, for each of these vertebrae, a measurement is made, on the CT image thereof, of the lateral distance from the patient""s midline to the point at which the insertion path intersects the skin of the patient""s back. By viewing the patient""s spine radiographically in the anterior-posterior direction, a marking is made on the skin of the patient""s back directly over the midline, and a transverse line is drawn for each of the vertebrae to be fixated over the centers of a pedicle thereof. Then, for each of the vertebrae to be fixated, an insertion point is established on the patient""s back, on the corresponding transverse line, at a distance equal to the lateral distance measured on the CT image thereof. Thereafter a guide pin is inserted through the patient""s back, and into a pedicle of each of the vertebrae to be fixated, through the insertion point established therefor and at the angle measured therefor.
Another problem with the method of percutaneous fixation as described in my prior patents is that the adapters that are attached to the proximal ends of the pedicle screws need to be available in a variety of sizes so that, during surgery, pedicle screws of the appropriate lengths can be selected to position the connecting link at the proper position just underneath the patient""s skin. Often the available adapters are either too long or to short to allow proper positioning of the connecting links. Moreover, during surgery, the attachment of the adapter to the screw is time consuming and at times difficult. Removal and reinsertion of adapters in an effort to find one of the proper length can also result in loosening and displacement of the pedicle screw. Still further objects of the invention, therefore, are to simplify the procedure for vertebral fixation by eliminating the need for various different sizes of adapters, to allow easier and more precise positioning of the connecting links, and to reduce the likelihood of loosening or displacement of pedicle screws.
In accordance with the invention, the step of inserting a guide pin through the patient""s back is followed by the step of introducing a lengthwise expansible pedicle screw, having proximal and distal ends, into the pedicle of each of the two or more vertebrae to be fixated. When each screw is properly introduced, its distal end is located inside the vertebra and its proximal end is located underneath the fascia of the patient""s back. Each screw is then expanded lengthwise until its proximal end is located adjacent to the fascia, but underneath the skin. The proximal ends of the pedicle screws are then rigidly connected together by connecting means located between the fascia and the skin of the patient""s back.
A preferred expansible pedicle screw in accordance with the invention comprises a shaft having distal and proximal portions. The distal portion has threads adapted to be threaded into the medullary canal of a vertebral pedicle, and the proximal portion has threads adapted to engage internal threads of a tubular extension. At its proximal end, the tubular extension has means, engageable by a wrench, for rotating the extension relative to the shaft in a direction to increase the distance between the proximal end of the tubular extension and the distal end of the shaft. Means are also provided for connecting a rigid link to said proximal end of the extension, whereby the extension of the expansible pedicle screw can be connected to another pedicle screw. Preferably, the shaft has a lengthwise internal passage for receiving a guide pin, and the tubular extension has a threaded female recess communicating with the passage for receiving a threaded adapter. A plurality of holes is preferably provided in an end face of the tubular extension for receiving projections of a wrench.
In the assembled vertebral fixation device, which comprises at least two expansible pedicle screws, the threads of the extensions are engaged partway with the threads of the shafts so that engagement of the link with the extensions prevents the extensions from rotating relative the shafts in directions such as to shorten the lengths of the expansible pedicle screws. The device therefore eliminates the need to keep multiple adapters of different sizes on hand during surgery, avoids the problem of selecting an adapter having the proper length, and allows precise positioning of the connecting link.
In various percutaneous spinal procedures, including decortication of vertebral plates, insertion of bone grafts, removal of herniated disc material and resection of nuclear tissue, it is often desirable to utilize two cannulae at the same time, one for insertion of manipulative instrumentation, e.g. forceps, and the other for endoscopic observation. My U.S. Pat. No. 5,395,317, dated Mar. 7, 1995, describes such percutaneous spinal procedures, and specifically relates to a technique in which a guide attached to a first cannula is used to maintain a second cannula in parallel relation to the first cannula.
Still further objects of this invention include the simplification of such percutaneous spinal procedures by obviating the second cannula, and the simplification of the procedure for gaining access to the intervertebral disc.
In accordance with the invention, a specially designed cannula assembly is used. The cannula assembly comprises a cannula having a wall of uniform thickness defining a cylindrical inner passage having an oval cross section, and an oval obturator located within the oval, cylindrical inner passage. The obturator has a cylindrical outer wall with an oval cross-section and closely fits the cylindrical inner passage of the cannula. The obturator preferably has an internal passage for receiving a guide, and has a blunt end projecting from the distal end opening of the cannula.
The cannula assembly is introduced percutaneously through the back of a patient, and the obturator is removed. After removal of the obturator, by virtue of the oval cross-section of the cannula passage, two or more instruments, for example an arthroscope and a forceps, can extend through the cannula simultaneously.
The introduction of the oval cannula is preferably carried out by inserting an elongated, hollow sleeve percutaneously toward an intervertebral disc of a patient posterolaterally, while the sleeve has an obturator extending through it; removing the obturator from the sleeve, while leaving the sleeve in place; passing over the sleeve an assembly comprising a cannula having an lumen with an oval cross-section and an obturator having an oval cross-section conforming to the lumen of the cannula, the oval obturator having a longitudinal passage for receiving the sleeve; removing the oval obturator from the cannula, while leaving the cannula in place.
A trephine may be inserted through the cannula toward the intervertebral disc, to form a fenestration in the annulus fibrosis of the disc, and the cannula can then pass through the fenestration. Preferably, the longitudinal passage in the oval obturator is large enough to receive the trephine. This allows the trephine to be operated while the oval obturator in still in place in the cannula, for more accurate guidance of the trephine.
The annular fenestration may be performed, under direct arthroscopic visualization and magnification, by a long and thin handle knife which is introduced into the lumen of the oval cannula.
Other objects, details and advantages of the invention will be apparent from the following detailed description when read in conjunction with the drawings.